R O C K Y
M O U N T A I N
I N S T I T U T E
Home Energy Brief
#2 WINDOWS
Windows are typically the weakest link in a building’s thermal barrier. In cold climates, they’re responsible for 10–25% of a home’s winter heat loss, or
$50–125 on an average annual heating bill. In
warmer climates, excess solar radiation entering
through windows can boost air-conditioning bills by
a similar amount.
Replacing windows is a costly proposition, but
there are many other low- and high-tech ways to
improve the performance of existing windows. The
first section of this brief highlights the most costeffective measures for minimizing energy loss through
windows. For those doing a major remodel or building a new home, the second section gives an introduction to high-performance window technologies.
With windows, as with all household energy-saving
measures, a good rule of thumb is to start with the
jobs that cost the least and yield the greatest savings.
A common-sense combination of three or four techniques should result not only in substantial savings on
your heating/cooling bill, but also in enhanced comfort: cold drafts will be minimized, the temperature of
your house will remain more constant, and areas near
windows will become more habitable.
RETROFITTING EXISTING WINDOWS
Nearly half of all residential windows in place are
single-pane, which do little more than prevent the
wind from blowing through homes. Even doublepane windows (which many local building departments now require in all new construction) can be
made more effective using the following techniques.
Cold-Weather Solutions
• The cheapest way to reduce heat loss is to stop the
wind from blowing in your windows. Caulk
around the edges of drafty frames and reputty
loose panes; install weatherstripping around windows that are opened often, and use rope caulk
(putty-like material that comes pre-formed in
strips or coils) to seal windows that will remain
shut for the winter.
• The next best option after caulking is to install
temporary plastic barriers to create an extra insu-
WINDOW PERFORMANCE
Heat is lost through windows by direct conduction
through the glass and frame, by air leakage through and
around the window assembly, and by the radiation of
heat by room-temperature objects such as people and furniture. (In hot weather, the same processes work in
reverse, resulting in heat gain.) Conduction can be reduced
by various means, including double-glazing and storm windows (which create dead-air space between the panes),
curtains, and high-tech frame materials; caulk and putty
help minimize leakage; and curtains, shades, and special
light-filtering films cut down on radiation.
Windows are rated by either R-value (resistance to the
flow of heat) or U-value (ability to transfer heat).The higher the R-value, the more energy-efficient the window. U-values are the reverse: the lower the number, the better. R-2 is
the same as U-0.5, R-4 is the same as U-0.25, and so on.
Until recently, most windows were rated on their “center-of-glass” R- or U-value; such ratings ignore significant
heat loss through the edge of the glass and the frame. A
more useful measure is the “whole-unit” value. When
comparing brands or window types, make sure they’re
rated in the same way.
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lating dead-air space inside the window. Most
hardware stores carry inexpensive kits consisting
of heat-shrink clear plastic (you provide the heat
with a hair dryer). Expect to pay about 20–40¢
per square foot—well worth it even for just one
winter season, since such plastic barriers can cut
the heat loss through single-pane windows by
25–40%.
If you own storm windows, repair and weatherize
them as you would regular windows. Storm windows are more expensive ($7.50–$12.50 per
square foot of window area) than temporary barriers, but they have the advantages of permanence
and better performance (up to 50% reduction in
heat loss).
Install tight-fitting insulating shades. These
shades incorporate layers of insulating material, a
radiant barrier, and a moisture-resistant layer to
help prevent condensation. The whole unit rolls
up during the day, and to reduce air flow should
be sealed against the window frame at night.
While conventional curtains are never as tight as
shutters or insulating shades, you can use tightly
fitting valances (wood or metal frames that go
over the top of existing curtains) to keep air from
circulating behind the curtains and creating a
chill. During the day, be sure to open curtains on
south-facing windows to let in heat and light.
Clean solar-gain windows. Note also that remov-
ing outside screens for the winter on south windows can increase solar gain by up to 40%.
Warm-Weather Solutions
• Hang white window shades or blinds, which can
reduce solar heat gain by 40–50%.
• Close south- and west-facing curtains during the
day, and keep the windows closed.
• Install awnings on south-facing windows where
there’s no roof overhang to provide shade. Canvas
awnings are more expensive than window shades,
but they’re more pleasing to the eye, they work
better, and they don’t obstruct your view.
• Alternatively, hang tightly-woven insect screens
or bamboo shades outside windows during the
summer. They’ll reduce your view, but they’ll stop
60–80% of the sun’s heat from getting to the window.
• Plant trees or build a trellis to block out solar
radiation. Deciduous (leaf-bearing) trees planted
to the south, east, and west of your building provide valuable shade in the summer, then drop
their leaves in the winter to allow half or more of
the sun’s heat to warm you on clear winter days.
• Apply retrofit window films (see below) to the
inside of east-, south-, and west-facing windows
and glass doors.
COST VS. R-VALUE FOR DIFFERENT WINDOW TYPES
(All are retail prices for 3x5-foot casement window)
Whole unit
R-value
Single-pane, wood frame
Double-pane, wood frame
Double-pane low-e, wood frame
Double-pane low-e, gas fill, wood frame
Double-glass, plus suspended Heat Mirror
Triple-glass, two low-e coats, gas fill, vinyl frame*
Double-glass, plus two films, gas fill, wood frame
1.1
2.0
2.3
2.6
3.1
4.5
4.8
Adapted from Space Heating Technology Atlas (1993), E SOURCE, Boulder, CO.
Window prices vary greatly by manufacturer, frame type, and window type.
* New Viking Industries triple pane; only available in 13 western states as of November 1994.
Retail
price
$190
$205
$240
$240
$270
$225
$360
Cost per
square foot
$13
$14
$16
$16
$18
$15
$24
HIGH-PERFORMANCE WINDOWS
An ordinary, well-sealed double-pane window has
a whole-unit R-value of 2.0. Compare that with the
high-performance windows that have become available in the past decade, which provide insulation of
up to R-5 or more.
Unfortunately, while most high-performance windows cost only 20–50% more than standard doublepane units (see table), the payback period for such an
upgrade in an existing house is typically 15–20 years.
For most people, that’s too long to be cost-effective.
However, if you’re planning to replace windows anyway, you’ll recoup the extra cost of high-performance
models in just a few years. In new construction, these
windows can help pay for themselves further by
reducing the size (and hence the cost) of heating and
cooling systems required.
one—however, the higher conductivity of the metal
frame can be offset by a “thermal break” made of
highly insulating material built into the seal and/or
the glazing spacer to reduce heat loss. Also check a
window’s air tightness rating. Look for units with a
rating no higher than 0.2 cfm/ft (cubic feet of air
leakage per minute per foot of window edge). The
best windows are lower than 0.1 cfm/ft.
Low-Emissivity (Low-e) Coatings
Windows equipped with low-emissivity (low-e)
coatings allow visible light through but selectively
block infrared radiation (heat). That means heat has
a much harder time escaping on cold days and entering on hot ones, boosting insulating efficiency.
Some manufacturers make windows “tuned” to hot
or cold climates. The basic difference is that hot-climate windows block more solar radiation to reduce
cooling costs, whereas cold-climate windows admit
more solar radiation to lower heating costs.
Gas Fills
The insulating value of a low-e window can be
improved by 15–20% with low-conductivity argon
or krypton gas sealed inside the window instead of
air. A number of manufacturers are now gas-filling
many of their models. Though more expensive, krypton insulates better than argon, allowing the window
assembly to be thinner.
Don’t worry: these are inert gases that occur naturally in the atmosphere, and are harmless even if the
window breaks.
Frames and Edge Seals
Now that glazings have advanced so much, reducing heat flow through frames has become more
important. Good edge seals are also necessary to
keep moisture out and gas fills in. All things being
equal, a wood or vinyl frame will outperform a metal
Cross-section of a superwindow. Adapted from an illustration
by Hurd Millwork.
Superwindows
State-of-the-art superinsulating windows, or
superwindows, combine all the above advanced features (low-e coatings, gas fill, good edge seals, insulated frames, and airtight construction) and then go
one better: The low-e coating is applied not to the
glass but to one or two sheets of Southwall Corp.’s
Heat Mirror™, a transparent polyester film suspended between the glass panes. The result is windows
with whole-unit ratings of up to R-6 (whole unit)—
about twice as efficient as the same thickness of fiberglass.
Heat Mirror comes in different “flavors” to block
out varying amounts of solar radiation. For example,
Heat Mirror 66 is used for hot climates and blocks
more infrared, while Heat Mirror 88 is designed to
allow more solar gain. Several manufacturers (such as
Hurd, Pella, and Visonwall) use Heat Mirror to make
their superwindows. Viking’s new triple-pane window doesn’t, but it has a whole-unit rating comparable to the best superwindows, and is priced competitively with regular low-e units.
Because of their extra cost, superwindows are most
likely to be cost-effective in very cold climates.
Superwindows generally don’t earn their keep in hot
climates, where you’re better off buying less expensive
low-e windows and using shading techniques.
Retrofit Window Films
Retrofit window films can be installed directly to
an existing window to cut glare and block heat in
hot, sunny situations (they are not typically used in
cold climates). Most retrofit films need to be installed
by a professional and are fairly expensive—ranging
from $1 to $5 per square foot of window area—but
they still cost far less than new windows. Most of
these films are tinted, although Southwall’s Heat
Mirror XIR is essentially transparent.
RESOURCES
Information
American Architectural Manufacturers Association,
1540 E. Dundee Rd., Palatine, IL 60067 (708/202-1350).
Publishes Window Selection Guide and Windows: A Consumer
Guide.
Energy Efficiency & Renewable Energy Clearinghouse,
PO Box 3048, Merrifield, VA, 22116 (800/523-2929). Free
pamphlets on high-performance windows and retrofit
options.
Iris Communications, Inc., 258 E. 10th Ave., Suite E,
Eugene, OR 97401-3284 (800/346-0104). Has a 20minute video, Window Design Decisions, for owner-builders,
architects, and home-buyers ($20).
National Fenestration Rating Council, 1300 Spring
Street, Suite 126, Silver Spring, MD 20910 (301/589-6372).
Publishes a Certified Product Directory (available for $10) and
has a video for architects and builders.
National Wood Window and Door Association, 1400
East Touhy Avenue, Des Plaines, IL 60018 (708/299-5200).
Publishes A Guide to Energy-Saving Windows.
Window Manufacturers
Look in the Yellow Pages under “Windows.” It’s
best to order windows locally because they’re heavy
and fragile and don’t ship well, and because local
window representatives will know what type of window works best in your climate. Some names to look
out for: Alenco, Anderson, Better-Bilt, Blomberg,
Craftline, H Window, Hurd, Kolbe & Kolbe,
Marvin, Milgard, Northwest, Peachtree, Pella, Pozzi,
Viking, Visionwall, Weather Shield, and Winter Seal.
Glazing Manufacturers
The following companies, which together account
for about 80% of the high-performance glass market,
can provide consumer information packets.
Cardinal Insulated Glass, 12301Whitewater Dr.,
Minnetonka, MN 55343 (612/935-1722).
Libby Owens Ford, 811 Madison Ave., Toledo, OH
43695 (419/247-4884).
PPG Industries, 1 PPG Place, Pittsburgh, PA 15272
(412/434-2858).
Other Manufacturers
Appropriate Technology Corporation, Box 975,
Brattleboro, VT 05302 (800/257-4501). Manufactures insulated track-mounted window shades.
Courtaulds Performance Films, PO Box 5068,
Martinsville, VA 24115 (800/345-6088). Makes retrofit
window films.
3M, Woodbury, MN (800/364-3577). Makes tinted and
air-gap window films.
Southwall, 1029 Corporation Way, Palo Alto, CA 94303
(800/365-8794). Manufacturer of Heat Mirror.
For more on window technologies, insulating solutions, and related home-energy measures, see Rocky Mountain Institute’s forthcoming Homemade Money: How to Save Energy and Dollars in You Home. Other titles in this series of Home Energy Briefs include Lighting;
Water Heating; Refrigerators and Freezers; Cooking Appliances and Diswashers; Washers, Dryers, and Miscellaneous Appliances; and Computers
and Peripherals. Written by Richard Heede, Maureen Cureton, and Dave Reed. © Rocky Mountain Institute 1995.
Rocky Mountain Institute is a nonprofit research and educational organization whose goal is to foster the efficient and sustainable
use of resources as a path to global security. This publication is intended to help you improve the resource efficiency of your home.
You should use your best judgement about your home, and seek expert advice when appropriate. Rocky Mountain Institute does not
endorse any products mentioned and does not assume any responsibility for the accuracy or completeness of the information in this
Brief. These Briefs are funded by the educational Foundation of America and the W. Alton Jones Foundation. Printed on recycled
paper with soy-based inks.
To request additional copies please contact Rocky Mountain Institute, 1739 Snowmass Creek Road, Snowmass CO 81654-9199
Telephone: (970) 927-3851 Fax: (970) 927-3420